The invention relates to a rotary embossing machine.
In rotary embossing machines according to the preamble by means of a material layer to be embossed, guided at a constant speed through the embossing gap, an embossing foil or sheet web is so moved that during an embossing interval it travels at the same speed as the material layer. This is necessary so that the embossing material, e.g. discreet, successive embossing units such as pictures or texts, or a part of an ink film to be embossed, can pass in undistorted/unsmeared manner onto the material layer to be embossed and consequently the embossing foil or sheet web does not tear during the embossing interval.
In the case of embossing foil webs with discreet embossing foil units a very good registration stability, i.e. a positional accuracy of the embossing unit with respect to the intended embossing point is sought. However, with embossing foils with ink films the aim, for minimizing waste with unused ink film areas, is to obtain a minimum spacing between succeeding ink film areas still to be embossed from the preceding, ink film areas already removed by embossing.
In order to save material, the aim is for successive ink film areas to be embossed and generally also successive embossing units on the embossing foil web closer together than successive embossing locations on the material layer. This makes it necessary for the embossing foil web to be guided more slowly outside the embossing interval than the material layer, but before the embossing interval is accelerated to the material layer speed and is then decelerated again and optionally also retracted. These speed changes are brought about by foil accelerating means, this foil acceleration being understood to mean both a speed rise and a speed drop, as well as a direction reversal of the foil web movement.
A rotary embossing machine with feed speed increase and decrease operating according to the hot embossing process is disclosed in DE-OS 29 31 194. As a result of the high accelerations/decelerations required in particular at high operating speeds, the in certain circumstances relatively tension-sensitive embossing foil web can be exposed to strong stresses, which can distort the web up to tearing and can therefore reduce the embossing quality and give rise to operating faults. For extending the acceleration paths and therefore for reducing the degree of acceleration and tensile loading in the longitudinal direction, it is proposed in DE 37 13 666 A1 to retract the embossing foil web prior to each new embossing interval, so as to permit a longer "starting distance" and also a longer "braking distance".
As is e.g. shown in EP 415 881 A2, forward and reverse movements of paper webs relatively insensitive to longitudinal tensile stresses are already used for register and print length correction in printing presses. Guide rollers upstream and downstream of the printing gap controllable with respect to the rotation speed and direction ensure the paper web advance and return by means of partly briefly mutually displaced movements coordinated by means of a complicated control means and there can be brief web tension peaks or also a complete failure of the web tension.
The aforementioned embossing foil web-typical risks of distortion or tearing of the web in the case of acceleration processes could admittedly be reduced by slowing down the acceleration paths in the described way. However, the direct adoption of solution suitable in certain circumstances for paper web guidance for sensitive embossing foils or sheets could, if at all, only be successful with considerable control equipment costs. Foil stability problems can in particular occur with thinner embossing sheets, which are more sensitive to optionally even short, pronounced longitudinal stressing during the acceleration processes.